Conventional metallic materials intended for high-temperature use are limited, for practical service, to metal temperatures below a temperature of 1800.degree. to 2000.degree. F. (982.degree. to 1093.degree. C.). Beyond this temperature, the available advanced iron-, nickel-, and cobalt-base alloys do not possess adequate strength for most applications. Hence, for service at temperatures beyond 2000.degree. F. (1093.degree. C.), ceramic materials and refractory metals have to be considered.
Most ceramic materials possess excellent resistance to oxidation and corrosion by high-temperature environments. In addition, they have good high-temperature creep strength. However, ceramic materials have a major limitation to their use as structural materials because of their brittleness. Brittleness can result in unpredictable, catastrophic failure and reduced reliability. Additionally, there are problems in ceramic-to-ceramic and ceramic-to-metal joining.
In comparison, refractory metals possess excellent high-temperature strength and generally have good ductility and toughness. They are weldable and can be joined to produce long sections. However, limitations arise from the fact that the refractory metals do not possess any practical resistance to catastrophic attack by high-temperature oxidizing environments.
Attempts have been made in the past to protect refractory metals from catastrophic oxidation in high-temperature oxidizing environments. Attempts at conventional alloying to promote protective oxide scale formation have been notably unsuccessful. Alloying requires additions of large amounts (approximately 30 percent) of the protective oxide-forming elements (such as silicon and aluminum). Such additions lead to drastic lowering of the melting temperature of the refractory metals and thus, loss of high-temperature strength. Coatings rich in aluminum and/or silicon (aluminide and silicide coatings) have been developed, which, when present on the surface of the refractory metal, oxidize and form protective Al.sub.2 O.sub.3 and/or SiO.sub.2 scales. However, diffusion of aluminum or silicon from the coatings into the substrate often occurs and this eventually has adverse effects on the mechanical properties of the substrate, as well as depleting the protective coating. Although extensive research has been done in the past to optimize coating compositions and to develop barrier layers to minimize the effects of interdiffusion, the latter objective has never been properly realized. Another major disadvantage of these coatings is that even pinholes in the coating can lead to rapid destruction of the substrate being protected.
It has been attempted to provide high temperature alloys by compositions of molybdenum, nitrogen and silicon as exemplified in U.S. Pat. No. 3,110,590 to Little. The advantages of the present invention however are not obtained therein since elemental molybdenum and silicon are not present to any appreciable extent. In the present invention the metallic characteristics of molybdenum are preserved while oxidation resistance is enhanced.
Another method of providing high temperature oxidation resistance for niobium has been attempted by forming a coating consisting of one or more stable compounds using nitrides as in British Patent No. 942,853 to Pokorny. This however is a surface layer over the metal itself and does not protect in the manner of the present invention. Likewise, U.S. Pat. No. 4,492,522 to Rossmann provides for a surface coating of titanium nitride.